The Iron Hydrido Complex [FeH(dppe)2]ϩ
a Mattson Genesis Type I spectrometer. Far-infrared (FIR) spectra
were measurd using a Bruker IFS66 spectrometer. Raman spectra
with an irradiation at 1060 nm were recorded on a Bruker IFS 66/
CS NIR FT-Raman spectrometer. The setup involves a 350 mW
Nd-YAG-Laser with an excitation wavelength of 1064 nm. Samples
were pressed as neat compounds into the groove of a sample holder
which was sealed with a glass plate to ensure inert gas conditions.
Raman spectra with excitation in the visible light range were per-
formed on a DILOR XY spectrometer equipped with a closed-
cycle cryostat and a CCD detector.
Another sensitive probe of the electronic and geometric
structure of these systems is provided by the spectroscopic
signatures of the Fe-H interactions. Upon coordination of
N2 in trans position the Fe-hydride interaction is weakened,
which is evident from the lowering of the Fe-H stretching
vibration. Also in terms of Fe-H stretching frequencies,
complex 2b is intermediate between 1 and 2a, supporting
the above conclusions. Fe-H bonding is further reflected by
intense low-energy transitions visible in the electronic ab-
sorption spectrum which we attribute to hydride Ǟ Fe CT
transitions. For dinitrogen and thf coordinated
[Fe(H)(dppe)2]ϩ the hydride Ǟ Fe CT transition is found
at ϳ19000 and ϳ15000 cmϪ1, respectively, whereas in para-
magnetic, solid [FeH(dppe)2]BPh4 it is located at
Synthesis of [FeH(dppe)2]BPh4 (1). This complex was prepared fol-
lowing the literature procedure [14]. Elemental Analysis: Theor. C
77.8, H 5.9; Found C 78.0, H 6.2 %.
ϳ17000 cmϪ1
.
Synthesis of [FeH(N2)(dppe)2]BPh4 (2a/b). (a) Compound 2a was
initially synthesized by the following method: A solution of 400 mg
(0.341 mmol) of 1 in 30 mL of thf was stirred for 3 hours under
dinitrogen; the colour thereby changed from blue-green to red. The
solvent was removed in vacuo and the residue taken up with 10 mL
of dichloromethane. The remaining solid was removed by filtration
and washed twice with 2 mL of dichloromethane. The combined
filtrates were reduced by vacuum to a volume of 3 mL; sub-
sequently 20 mL of diethyl ether were added slowly. The red pre-
cipitate was collected by filtration and dried in vacuo. Yield of 2a:
170 mg (42 %). For the present study compound 2a was prepared
by the method described in (b) which was found to be superior
to (a).
In summary, the electronic structure of the FeII hydrido
complex [FeH(dppe)2]ϩ (1) has been defined and a detailed
understanding of its reactions with N2 leading to the yellow
and green dinitrogen adducts 2a and 2b has been achieved.
This completes our investigations on the binding of N2 to
FeII diphosphine complexes with different trans ligands X
(XϭH, Hal). Notably, three types of reactions with N2
have been established: spin-allowed Sϭ0 Ǟ Sϭ0 for
[FeH(dppe)2]ϩ
spin-allowed Sϭ1
Ǟ
[FeH(N2)(dppe)2]ϩ in solution;
Sϭ1 for [FeH(dppe)2]ϩ
Ǟ
Ǟ
[FeH(N2)(dppe)2]ϩ in the solid state and spin-forbidden Sϭ
1 Ǟ Sϭ0 for [FeX(depe)2]ϩ Ǟ [FeX(N2)(depe)2]ϩ, XϭCl,
Br. This class of compounds thus provides a textbook ex-
ample for the fact that simple changes in the coordination
sphere of transition-metal complexes can greatly influence
their electronic structure and, correspondingly, their reac-
tivity.
(b) A suspension of [FeH(dppe)2]BPh4 (1) (0.30 g, 0.25 mmol) in
10 mL of toluene was allowed to stir overnight at room temperature
under 1 atm of nitrogen. The resulting yellow solid (2a) was sepa-
rated by filtration and dried under vacuum. Elemental Analysis:
Theor. C 76.0, H 5.8, N 2.3; Found C 74.3, H 5.5, N 1.9 %.
(c) [FeH(dppe)2]BPh4 (1) reacts with nitrogen in the solid state,
resulting in a green compound (2b) after storing under 1 atm of
nitrogen for several weeks. Elemental Analysis: Theor. C 76.0, H
5.8, N 2.3; Found C 73.7, H 5.8, N 0.9 %.
Experimental Section
Synthetic Procedures and Spectroscopic Measurements. Synthesis
and handling of all compounds were performed under Argon or
N2 by use of Schlenk techniques and gloveboxes. All solvents were
dried and freshly destilled under argon. The ligands 1,2-bis(diethyl-
phosphino)ethane (depe) and 1,2-bis(diphenylphosphino)ethane
(dppe) were obtained from Strem Chemicals; 15N2 (98 %) was pur-
chased from EURISO-TOP GmbH. All reagents were used without
further purification. Sample manipulations for vibrational, UV/
Vis- and NMR spectroscopy were performed in a glovebox.
Mössbauer spectra were recorded with a WISSEL setup equipped
with a He flow-through cryostat (Oxford CF506). Isomer shifts are
given with respect to α-iron. Magnetic susceptibility was measured
with a Physical Instruments (PI) vibrating sample (Foner) magnet-
ometer at the Institute of Inorganic and Analytical Chemistry, Uni-
versity of Mainz. Susceptibility data were corrected for diamagnetic
contributions using Pascal’s constants. Susceptibility measurements
in solution were performed by the Evans method. NMR spectra
were recorded on a Bruker Avance 400 pulse Fourier transform
spectrometer operating at a 1H frequency of 400.13 MHz (31P
161.98 MHz, 15N 40.55 MHz) using a 5 mm inverse triple-reson-
ance probe head. References as external standards: H3PO4 85 %
pure, δ(31P) 0 ppm; CH3NO2 neat, δ(15N) 0 ppm. UV-vis spectra
were measured with a Varian CARY 5 UV-vis-NIR spectrometer.
Middle-infrared (MIR) spectra were obtained on RbI pellets using
Acknowledgments. Financial support for this research has been
provided by DFG and FCI. Assistance in the vibrational measure-
ments bei Uschi Cornelissen is gratefully acknowledged. We thank
V. Ksenofontov at Johannes Gutenberg University Mainz for per-
forming the magnetic susceptibility measurements.
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